Kraftarm

Kraftarm, in physics and engineering, is the lever arm or moment arm of a force. It is defined as the perpendicular distance from the axis of rotation to the line of action of the force. The Kraftarm determines the rotational effect the force can produce.

Torque, or moment, is given by τ = F × r_perp, where F is the magnitude of the force and r_perp is the Kraftarm—the component of the position vector that is perpendicular to the force. If φ is the angle between the force and the lever arm, r_perp = r sin φ, and τ = F r sin φ. When the force is perpendicular to the lever (φ = 90 degrees), the Kraftarm equals the full lever length, and τ = F × r. When the force is parallel to the lever, τ = 0.

In practice, the Kraftarm is used to quantify mechanical advantage in levers, and in biomechanics to compute joint torques from muscle forces. The longer the Kraftarm for a given force, the larger the torque, within physical limits and allowing for changes with angle or geometry.

The term is used in various fields, including mechanical design, robotics, and structural analysis, to describe how effectively a force can produce rotation. It is distinct from the straight-line distance from the pivot; it is specifically the perpendicular distance to the line of action of the force.